Battery electrode and method of making the same

ABSTRACT

A battery electrode composed of a principal active material and a secondary active material and the method of making the same, which electrode will achieve the discharge potential characteristic of the secondary active material wherein the sole electronic path for discharge of principal active material is through the secondary active material. The discharge product of the secondary active material must be readily oxidized by the principal active material.

United States Patent Soto-Krebs [15] 3,655,450 .451 *Apr. 11,1972

[54] BATTERY ELECTRODE AND METHOD OF MAKING THE SAME [63] Continuation-impart of Ser. No. 445,904, Apr. 6,

[52] 11.8. CI ..l36/l07, 136/111 [51] Int. Cl. ..H0lm 21/00 [58] Field ofSearch ..136/l07, 111,30, 83, 102,

[56] References Cited UNITED STATES PATENTS 2,542,710 2/1951 Ruben ..136/107 2,795,638 6/1957 Fischbach ..l36/120 FOREIGN PATENTS OR APPLICATIONS 835,086 5/1960 Great Britain Primary Examiner-Anthony Skapars Att0mey-Alfred J. Snyder, .lr., Robert H. Robinson, Raymond L. Balfour and Anthony J. Rossi [5 7] ABSTRACT A battery electrode composed of a principal active material and a secondary active material and the method of making the same, which electrode will achieve the discharge potential characteristic of the secondary active material wherein the 'sole electronic path for discharge of principal active material is through the secondary active material. The discharge product of the secondary active material must be readily oxidized by the principal active material.

6 Claims, 8 Drawing Figures PATENTEDAPR 1 1 m2 SHEET 1 UF 2 Fig? Time Hours Time Hours Fig 3 n... n l 23 2:;

Time Hours SHEET 2 0F 2 Time Hours PATENTEDAPR 11 me o 5 D o 23 moozo BATTERY ELECTRODE AND METHOD OF MAKING THE SAME CONTINUATION-IN-PART APPLICATION This patent application is a continuation-in-part of my pending US. Pat. application, Ser. No. 445,904 filed Apr. 6, 1965.

BACKGROUND OF THE INVENTION Certain battery applications require a substantially constant voltage during discharge. Other applications demand a particular discharge voltage. Most applications dictate that the battery supply a maximum capacity while simultaneously restricting its size and weight. The problem of furnishing a battery for a given application is complicated by the fact that while some electrode active materials have inherently good capacities, they discharge at two or more potentials. Moreover, those materials which are most active and hence most desirable for battery use characteristically discharge at several voltage levels, thus making their use impracticable. Still other active materials which have good capacities and substantially constant discharge voltages, discharge at potentials which may not be suitable for a particular application.

It is, accordingly, a general object of the present invention to provide a new and improved electrode construction characterized by an increase in capacity per unit weight and volume.

It is another object of the present invention to provide electrode means for utilizing an active material at the lower potential of a second active material.

It is still another object of the present invention to provide electrode means for achieving a single potential discharge from a multivalent oxide, such as divalent silver oxide, that discharges at two or more potentials.

It is a further object of the present invention to provide an electrode having two different active materials which will utilize the capacity of both materials at the potential of the lower potential material.

It is still a further object of the present invention to provide a new and improved button-type cell construction and method for making the same.

SUMMARY OF THE INVENTION While not limited thereto, the principle of the present invention can be illustrated by means of a divalent silver oxide electrode which discharges at two different potentials. During the initial discharge of such an electrode, the divalent silver oxide is reduced to monovalent silver oxide. Theoretically, this reaction proceeds until all of the divalent silver oxide has been reacted. Next, the monovalent silver oxide is further reduced to metallic silver. The first reaction, while divalent silver oxide is present, provides an open circuit potential, in an alkaline electrolyte of approximately 1.8 volts vs. zinc. The second reaction while monovalent silver oxide is present, provides an open circuit potential in an alkaline electrolyte of approximately 1.6 volts vs. zinc.

In practice, due to such effects as polarization and the masking of the divalent oxide by the formation of monovalent oxide, before a divalent silver oxide cell is halfway through its useful life, its output voltage will drop 0.2 volts. Many types of battery-operated electronic equipment cannot tolerate a voltage change of this magnitude. A divalent silver oxide electrode in accordance with the present invention, however, will deliver all of its capacity at the monovalent potential, thus providing a substantially constant output voltage.

To achieve a lower potential discharge from an active material in accordance with the present invention, three conditions must be met. First, the discharge product of the lower potential active material must be readily oxidizable in the battery electrolyte by the higher potential active material. Second, the higher potential active material must be in electronic contact with the lower potential material. Third, the discharge circuit must have electronic connection only with the lower potential active material.

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Divalent silver oxide and monovalent silver oxide are examples of active materials which satisfy the first condition stated above. The discharge product of the lower potential active material, monovalent silver oxide, is metallic silver. Metallic silver is readily oxidizable by the higher potential active material, divalent silver oxide, to monovalent silver oxide. The second and third conditions are achieved in accordance with the present invention by a novel electrode structure utilizing as the principal active material a body of divalent silver oxide having thereon as the secondary active material a layer of monovalent silver oxide. The electrode contact means, that is, the electrical path for the discharge circuit is in contact only with the monovalentsilver oxide layer.

While the present invention has been described hereinbefore in connection with divalent silver oxide and monovalent silver oxide, the principles are applicable to other electrode active materials. Generally stated, the present invention provides a method of operating a battery at a lower potential than that normally developed by the principal active material. This method comprises withdrawing current from the principal active material through a current path consisting solely of an interposed layer of a secondary active material of a lower potential having a discharge product which is readily oxidized by the principal active material. The lower potential achieved from the electrode is that developed by the interposed layer of the secondary active material. Stated another way, the method of the present invention comprises maintaining electronic contact during discharge between the principal active material and the discharge circuit only through the secondary active material.

By way of example of the applicability of the present invention to other active material, manganese dioxide can be discharged at the lower potential of copper oxide. Similarly, divalent silver oxide can be discharged at the potential of copper oxide. Potassium permanganate can be discharged at a lower potential such as that of monovalent silver oxide or copper oxide. These examples are illustrative of only a few electrode combinations which are possible by means of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a curve showing the theoretical discharge characteristics of a silver-zinc primary cell using as the active material of the positive electrode monovalent silver oxide;

FIG. 2 is a curve showing the theoretical discharge characteristics of a silver-zinc primary cell identical to the primary cell of FIG. 1 but using as the positive active material an equal volume of divalent silver oxide;

FIG. 3 is a curve showing the theoretical discharge characteristics of a silver-zinc primary cell identical to the cells of FIGS. 1 and 2, but using a positive electrode in accordance with the present invention;

FIG. 4 is a cross-sectional view of a primary silver-zinc cell having composite positive silver electrode in accordance with the present invention;

FIG. 5 is a top elevation taken along the line 5-5 of FIG. 4;

FIG. 6 is a cross-sectional view of an electrode in accordance with the present invention wherein the isolating layer of secondary active material is formed by the in situ reduction of the principal active material by the metal of the inner surface of the electrode cup;

FIG. 7 is a partial sectional elevation taken along the line 7-7 of FIG. 6 and enlarged; and

FIG. 8 is a curve showing the discharge characteristics of an actual primary silver-zinc cell in accordance with the present invention compared to the discharge characteristics of another cell of the same type but incorporating a positive electrode comprising an equal volume of monovalent silver oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As indicated hereinbefore, both the broad and specific features of the present invention can be illustrated in connection with a positive silver electrode. The capacity in milliampere hours per gram and the specific gravity of both divalent and monovalent silver oxide are given in the following table:

As shown, divalent silver oxide has 1.87 times more capacity per gram than the monovalent oxide and has 1.95 times more capacity per unit volume than the monovalent oxide. The significance of this becomes apparent with reference to FIGS. 1 and 2. FIG. 1 shows a theoretical discharge curve of a conventional silver-zinc primary cell using monovalent silver oxide as the positive active material. FIG. 2 shows the theoretical discharge curve of a similar cell utilizing an equal volume of divalent silver oxide as the positive active material. Both of the discharge characteristics shown represent continuous discharges through a 300 ohm load at 73 F. Obviously, the cell of the FIG. 2, the cell utilizing divalent silver oxide as the positive active material, has substantially more usable capacity than the cell in FIG. 1. However, while the divalent silver oxide cell has significantly more capacity than the monovalent silver oxide cell, its discharge is characterized by two distinct voltage plateaus. One, at approximately 1.72 volts and the other at approximately 1.5 volts. Many battery applications, particularly transistorized devices such as hearing aids, cannot tolerate a voltage drop such as is exhibited by the cell of HG. 2.

The electrode structure of the present invention is designed to provide means for utilizing the capacity of an inherently high capacity material such as divalent silver oxide at the potential of a second material having a lower potential such as monovalent silver oxide. Referring to FIG. 3, there is shown the theoretical discharge curve of a silver-zinc cell identical to those shown in FIGS. 1 and 2, except that it utilizes an electrode of the present invention. The curve of FIG. 3 is plotted for discharge conditions identical with FIGS. 1 and 2. As shown by this curve, the cell exhibits a single potential discharge at the monovalent oxide potential level while utilizing the capacity of divalent silver oxide at that lower potential.

Referring now to FIG. 4, there is shown a sectional elevation of a silver-zinc primary cell, designated by the numeral 1, having a positive electrode in accordance with the present invention. The cell 1 is conventional in all respects with the exception of the construction of the positive electrode. The cell 1 has a two-part container comprising an upper section or cap 2 which houses the negative electrode, and a lower section or cup 3 which houses the positive electrode. As shown, the bottom cup 3 is formed with an annular shoulder 4 having a flange 5 which is crimped inward during assembly to seal the cell. The bottom cup 3 may be made of nickel plated steel, and the cap 2 may be made of tin plated steel. The cap 2 is insulated from the cup 3 and the flange 5 by means of a grommet 7 which is compressed between the cap 2 and the flange 5 during the crimping operation of cell assembly to provide a compression seal between these parts. The grommet 7 may be made of a suitable resilient electrolyte-resistant material such as neoprene.

The negative electrode of the cell 1 comprises a lightly compacted pellet 8 of finely divided amalgamated zinc. The zinc electrode 8 is separated from the positive electrode by means of an electrolyte-absorbent layer 9 and a membrane barrier 10. The electrolyte-absorbent layer 9 may be made of electrolyte-resistant, highly absorbent substance such as matted cotton fibers. Such a material is available commercially under the trademark Webril. The barrier layer 10 may be a suitable semi-permeable material such as cellophane, or comprise a suitable organic carrier such as polyethylene or polyvinyl chloride having a polyelectrolyte homogeneously dispersed therethrough. Such a material is described and claimed in US. Pat. No. 2,965,697, issued Dec. 20, 1960 to J. C. Duddy.

The positive electrode of the cell 1 comprises, in accordance with the present invention, a first pellet ll of the divalent silver oxide which is surrounded on the bottom and side surfaces by a layer of monovalent silver oxide 12. The pellet ll of divalent silver oxide is the principal active material and comprises the majority of the active material in the electrode available for discharge. The layer of monovalent silver oxide 12 is the secondary active material. As shown in FIGS. 4 and 5, the layer 12 of monovalent silver oxide isolates the pellet of divalent silver oxide from all electronic contact with the bottom cup 3 which is the positive terminal of the cell and the electrode contact.

This electrode may be formed in a number of ways. For example, the pellet 11 may be formed by first pelletizing finely divided divalent silver oxide powder in a suitable die. This pellet may then be centered in a bigger pellet die, and finely divided monovalent silver oxide powder compressed around it to form the composite pellet of the type shown in FIGS. 4 and 5. It is possible also to form the electrode by pelletizing a suitable quantity of divalent silver oxide powder and then chemically reducing its surface to the monovalent oxide. In addition, where desired, the surface layer of the divalent silver oxide pellet can be reduced to metallic silver and that layer subsequently reoxidized to monovalent oxide. Electrochemical reduction can also be utilized to reduce the surface layer of a divalent silver oxide pellet to the monovalent oxide. It should be noted that the thinner the layer of monovalent silver oxide, the more divalent oxide can thus be included in the electrode increasing its capacity. However, under no circumstances should there be a discontinuity in the monovalent oxide layer which would provide direct electronic contact between the divalent oxide body and the electrode to discharge in the conventional manner with two voltage plateaus.

Referring now to FIG. 6, there is shown another embodiment of the electrode of the present invention in which the monovalent silver oxide layer is formed in situ by a chemical reaction between the divalent oxide pellet and a metallic coating on the electrode cup. Specifically, the inner surface of the cup 3 is provided with a layer 13 of a metal which will be oxidized by divalent silver oxide. As shown in FIG. 7, this reaction will leave a layer of the metal oxide 14 and a layer of monovalent silver oxide 15 between the divalent oxide pellet 11 and the metal layer 13 on the can 3. The layer 13 may be plated on the inner surface of the cup 3, or it may be in the form of a foil liner or a metallic laminate in intimate contact with the cup 3. When the divalent oxide pellet 11 is placed in the cup 3, it oxidizes the surface of the layer 13 to form thereon a film 14 of the oxide of that metal and simultaneously a layer 15 of monovalent silver oxide is reduced on the adjacent surface of the divalent oxide pellet 11. When this reaction is complete, the monovalent silver oxide layer 15 completely isolates the divalent oxide pellet 11 from all electronic contact with the metal oxide layer 14 and hence the cup 3. The presence of electrolyte absorbed throughout the pellet 11 will promote the reaction. By this method, it is possible to obtain a very thin monovalent oxide layer and, hence, to include in the cell the greatest possible amount of divalent silver oxide. Several metals have been found to be suitable for use as the layer 13 to produce the isolating monovalent silver oxide layer. These include zinc, cadmium, copper and lead.

The use of zinc for the layer 13 has several advantages in a silver-zinc cell system. First, it introduces no foreign ions into the cell. More important, however, the zinc oxide layer is readily formed in the presence of an alkaline electrolyte and provides an extremely low electrical resistance between the cup 3 and the pellet 11.

In manufacturing electrodes of the type where the isolating monovalent silver oxide layer is produced by an in situ reaction between a layer 13 and the divalent silver oxide pellet 11, care must be taken that the divalent silver oxide does not puncture or in any other manner disrupt the layer 13 and establish direct contact with the cup 3. This is particularly true with the case where the layer 14 is plated on a thin foil and the thickness of the layer is extremely thin. As stated hereinbefore, if directed electronic contact is established with the divalent silver oxide, the conventional two voltage plateau discharge will be obtained rather than the monovalent oxide potential discharge which characterizes the present invention.

The curves of FIG. 8 demonstrate the increase in capacity actually available from a cell in accordance with the present invention. In this figure, Curve A is the discharge curve of a conventional silver-zinc cell, and Curve B is the discharge curve for a cell having a positive electrode in accordance with the present invention. The cell construction utilized in both cells was that shown in FIG. 4. The negative electrodes of both cells comprised lightly compacted battery grade metallic zinc amalgamated with 14 percent mercury. The cells were sealed with neoprene rubber grommets. In both cells, the separation between the electrodes comprised a layer of Webril absorbent and a 3-mil layer of membrane made in accordance with the teachings of the aforementioned U.S. Pat. No. 2,965,697. The electrolyte absorbent layer was saturated with an electrolyte formulated by dissolving 100 grams of potassium hydroxide and 16 grams of zinc oxide in I00 cc. of water.

The positive electrode of Cell A comprises 1.73 grams of commercially available battery grade monovalent silver oxide compressed into a pellet 0.1 inch thick and 0.485 inch in diameter. The positive electrode of Cell B, the cell in accordance with the present invention, comprised a central pellet containing 1.2 grams of divalent silver oxide 0.085 inch thick and 0.387 inch in diameter. This central pellet was surrounded by 0.6 gram of monovalent silver oxide which was compressed to provide a composite pellet 0.1 inch thick and 0.485 inch in diameter. The divalent silver oxide mix from which the central pellet was compressed comprised 96 grams of a specially prepared low-gassing divalent silver oxide, and 4 grams of lead dioxide to which there was added 3.5 cc. of the electrolyte described hereinbefore for each 100 grams of the dry mix.

Both cells showed an open circuit voltage of between 1.58 to 1.59 volts and each had an impedance lower than 6 ohms. The curves of FIG. 8 are for a continuous discharge of the cells through a 300 ohms resistance at a temperature of 73 degrees F. As shown from these curves, C11 B, the cell in accordance with the present invention, exhibited approximately 30 percent more useful capacity than an identical size cell of the conventional monovalent silver oxide type. Of equal importance is the fact that this increase in capacity is achieved without the usual two voltage plateau discharge usually associated with divalent silver oxide.

Electrodes in accordance with the present invention have been constructed using electrode materials other than divalent and monovalent silver oxides. For example, divalent silver oxide has been discharged at the potential of 0.9 volts vs. zinc through a 300 ohms load at 73 F. by using cupric oxide as the second electrode material. The cupric oxide composite electrode was constructed by mechanically pressing finely divided cupric oxide around a pellet of divalent silver oxide. Another cupric oxide-divalent silver oxide electrode was constructed in a manner similar to the electrode in FIG. 6. In this electrode, the inner surface of the bottom cup of the cell was plated with copper which was subsequently oxidized to cupric oxide in the presence of potassium hydroxide electrolyte by the divalent silver oxide pellet.

Other electrode combinations are available. For example, manganese dioxide which has a potential in alkaline electrolyte vs. zinc of 1.5 volts may be discharged at the potential of cupric oxide. Such a manganese dioxide-cupric oxide electrode has been constructed and discharged in accordance with the present invention by first pelletizing a small pellet of manganese dioxide and then compressing around that pellet an TABLE II Primary Active Material Secondary Active Material Voltage v. Zinc Voltage V. Zinc Material (open circuit) Material (open circuit) MnO, 1.5 CuO l.l AgO L8 A o 1.6 AgO 1.8 CuO l.l KMnO. L8 Ago; 1.6 KMnO. 1.8 CuO 1.1

In each case the primary active material must be able to oxidize the discharge product of the secondary active material. If

the materials chosen meet this requirement and the primary active material is in electronic contact with the secondary active material while the discharge circuit has electronic connection only with the secondary active material as described, the electrode will discharge only at the lower potential of the secondary active material.

From the foregoing, it can be seen that by means of the novel method and the unique electrode structure described, there has been provided electrode means for achieving a single potential discharge from an electrode active material such as divalent silver oxide which discharges at two potentials. In addition, it is possible to utilize the capacity of an electrode active material at the lower potential of a second active material. Of equal importance is that the method and the electrode construction of the present invention provides the means for controlling within certain limits the discharge potentials of active materials. This not only provides flexibility in cell design, but in many instances also a means for achieving higher capacity cells than heretofore have been available. In considering the present invention, it also should be noted that, while zinc electrodes have been used as reference electrodes and for the negative electrode of the specific cells described,

this has been done for the purpose of illustration only and not by way of limitation. An electrode in accordance with the present invention may be utilized in cells where the electrode of the opposite polarity is of any conventional type.

Having thus described my invention, I claim:

1. In a battery having a positive electrode, a negative electrode and an electrolyte, the improvement comprising a positive electrode housed in a cathode cup, the principal active material of said positive electrode consisting essentially of a first metallic oxide, a secondary active material consisting essentially of a second metallic oxide interposed between said principal active material and said cathode cup as a continuous layer, said continuous layer of secondary active material being formed in physical and electrical contact with said principal active material prior to placing said principal active material in said cathode cup whereby said principal active material is physically isolated from the inner surface of said cathode cup and said principal active material is never disposed against the inner surface of said cathode cup, said secondary active material having a lower potential than said principal active material and a discharge product which, in the presence of the electrolyte, reacts with said principal active material to form secondary active material, and said battery having an output voltage throughout discharge characterized by the voltage of said secondary active material.

2. A battery in accordance with claim 1 in which the principal active material is divalent silver oxide and the secondary active material is monovalent silver oxide.

3. A battery in accordance with claim 2 in which the inner surface of said cathode cup is silver, copper or zinc.

4. In a battery having a positive electrode, a negative electrode and an electrolyte, the improvement comprising a positive electrode housed in a cathode cup, the principal active material of which consists essentially-of a first metallic oxide and the remaining active material of which consists of a continuous layer of a second metallic oxide in physical and electrical contact with said first metallic oxide, said layer of second metallic oxide being disposed against the inner surface of said cathode cup which inner surface is wet by said electrolyte, said first metallic oxide never being disposed against the inner surface of said cathode cup, and said battery being characterized throughout discharge by a potential characteristic of said second metallic oxide and said negative electrode in said electrolyte.

S. A battery in accordance with claim 4 in which the first metallic oxide is divalent silver oxide and the second metallic oxide is monovalent silver oxide.

6. A battery in accordance with claim 4 in which the inner surface of said cathode cup is silver, copper or zinc. 

2. A battery in accordance with claim 1 in which the principal active material is divalent silver oxide and the secondary active material is monovalent silver oxide.
 3. A battery in accordance with claim 2 in which the inner surface of said cathode cup is silver, copper or zinc.
 4. In a battery having a positive electrode, a negative electrode and an electrolyte, the improvement comprising a positive electrode housed in a cathode cup, the principal active material of which consists essentially of a first metallic oxide and the remaining active material of which consists of a continuous layer of a second metallic oxide in physical and electrical contact with said first metallic oxide, said layer of second metallic oxide being disposed against the inner surface of said cathode cup which inner surface is wet by said electrolyte, said first metallic oxide never being disposed against the inner surface of said cathode cup, and said battery being characterized throughout discharge by a potential characteristic of said second metallic oxide and said negative electrode in said electrolyte.
 5. A battery in accordance with claim 4 in which the first metallic oxide is divalent silver oxide and the second metallic oxide is monovalent silver oxide.
 6. A battery in accordance with claim 4 in which the inner surface of said cathode cup is silver, copper or zinc. 